A plant's phenotypic characteristics that enhance photosynthetic resource use efficiency may be controlled through a number of cellular processes. One important way to manipulate that control is by manipulating the characteristics or expression of regulatory proteins, proteins that influence the expression of a particular gene or sets of genes. For example, transformed or transgenic plants that comprise cells with altered levels of at least one selected regulatory polypeptide may possess advantageous or desirable traits, and strategies for manipulating traits by altering a plant cell's regulatory polypeptide content or expression level can result in plants and crops with commercially valuable properties. Examples of such trait manipulation include:
Increasing Canopy Photosynthesis to Increase Crop Yield.
Recent studies by crop physiologists have provided evidence that crop-canopy photosynthesis is correlated with crop yield, and that increasing canopy photosynthesis can increase crop yield (Long et al., 2006. Plant Cell Environ. 29:315-33; Murchie et al., 2009 New Phytol. 181:532-552; Zhu et al., 2010. Ann. Rev. Plant Biol. 61:235-261). Two overlapping strategies for increasing canopy photosynthesis have been proposed. The first recognizes great potential to increase canopy photosynthesis by improving multiple discrete reactions that currently limit photosynthetic capacity (reviewed in Zhu et al., 2010. supra). The second focuses upon improving plant physiological status during environmental conditions that limit the realization of photosynthetic capacity. It is important to distinguish this second goal from recent industry and academic screening for genes to improve stress tolerance. Arguably, these efforts may have identified genes that improve plant physiological status during severe stresses not typically experienced on productive acres (Jones, 2007. J. Exp. Bot. 58:119-130; Passioura, 2007. J. Exp. Bot. 58:113-117). In contrast, improving the efficiency with which photosynthesis operates relative to the availability of key resources of water, nitrogen and light, is thought to be more appropriate for improving yield on productive acres (Long et al., 1994. Ann. Rev. Plant Physiol. Plant Molec. Biol. 45:633-662; Morison et al., 2008. Philosophical Transactions of the Royal Society B: Biological Sciences 363:639-658; Passioura, 2007, supra).
Improving water use efficiency (WUE) to improve yield. Freshwater is a limited and dwindling global resource; therefore, improving the efficiency with which food and biofuel crops use water is a prerequisite for maintaining and improving yield (Karaba et al., 2007. Proc. Natl. Acad. Sci. USA. 104:15270-15275). WUE can be used to describe the relationship between water use and crop productivity over a range of time integrals. The basic physiological definition of WUE equates the ratio of photosynthesis (A) to transpiration (T) at a given moment in time, also referred to as transpiration efficiency. However, the WUE concept can be scaled significantly, for example, over the complete lifecycle of a crop, where biomass or yield can be expressed per cumulative total of water transpired from the canopy. Thus far, the engineering of major field crops for improved WUE with single genes has not yet been achieved (Karaba et al., 2007. supra). Regardless, increased yields of wheat cultivars bred for increased transpiration efficiency (the ratio of photosynthesis to transpiration) have provided important support for the proposition that crop yield can be increased over broad acres through improvement in crop water-use efficiency (Condon et al., 2004. J. Exp. Bot. 55:2447-2460).
Estimates of water-use efficiency integrated over the life of plant tissues can be derived from analysis of the ratio of the 13C carbon isotope to the 12C carbon isotope in those tissues. The theory that underlies this means to estimating WUE is that during photosynthesis, incorporation of 13C into the products of photosynthesis is slower than the lighter isotope 12C. Effectively, 13C is discriminated against relative to 12C during photosynthesis, an effect that is integrated over the life of the plant resulting in biomass with a distinct 13C/12C signature. Of the many steps in the photosynthetic process during which this discrimination occurs, discrimination at the active site of Rubisco is of most significance, a consequence of kinetic constraints associated with the 13CO2 molecule being larger. Significantly, the discrimination by Rubisco is not constant, but varies depending on the CO2 concentration within the leaf. At high CO2 concentration discrimination by Rubisco is highest, however as CO2 concentration decreases discrimination decreases. Because the CO2 concentration within the leaf is overwhelmingly dependent on the balance between CO2 influx through the stomatal pore and the rate of photosynthesis, and because the stomatal pore controls the rate of transpiration from the leaf, the 13C/12C isotopic signature of plant material provides an integrated record of the balance between transpiration and photosynthesis during the life of the plant and as such a surrogate measure of water-use efficiency (Farquhar et al. 1989. Annu. Rev. Plant Physiol. Plant Mol. Biol. 40:503-537).
Improving Abiotic Stress Tolerance to Improve Yield.
Field grown crops experience abiotic stress over a continuum of severity, dictated by complex interaction between discrete environmental factors and drivers of stress that include, but are not limited to, speed of onset, duration and severity. Screening for genes to improve abiotic stress tolerance has not typically been sophisticated enough to target many places along this stress continuum. One result of this screening limitation was described above, a tendency to identify leads for stress tolerance unsuited to field relevant stress. For productive acres this stress is typically mild and frequent and can manifest as a constraint to photosynthesis experienced during the stress only. This type of stress is in contrast to damage and downregulation of photosynthesis in response to severe stress on more marginal farming acres, that will compromise rates of photosynthesis long after the stress has been relieved. Importantly, phenotypes that would be expected to protect against severe stress would be expected to compromise yield under benign environmental conditions. Soil water conservation, through decreased stomatal conductance and protection against light damage, through decreased leaf chlorophyll content are two such phenotypes. Both could compromise photosynthesis, however this cost would be short lived on marginal acres, where these phenotypes would also be expected to limit the severity of the stress experienced and result in faster recovery from stress. Over repeated stress cycles canopy photosynthesis and yield would be expected to increase. Consequently the ability to protect against long term downregulation of, or damaged to, the photosynthetic apparatus is considered a pathway to improving crop yield on more marginal lands where crop yield can be impacted by severe weather events.
With these needs in mind, new technologies for yield enhancement are required. In this disclosure, a phenotypic screening platform that directly measures photosynthetic capacity, water use efficiency, nitrogen use efficiency, and stress tolerance of mature plants was used to discover advantageous properties conferred by ectopic expression of the described regulatory proteins in plants.